1. Field of the Invention
This invention relates generally to an apparatus and method for selecting a network upon power-up of a cellular phone, and more specifically to an apparatus and method for deriving the current network location of a cellular phone from the previous movement of the cellular phone within a Code-Division Multiple Access (CDMA) Cellular Mobile Telephone System (CMTS).
2. Related Art
A Cellular Mobile Telephone System (CMTS) is typically divided into multiple geographical areas known as systems. The systems are sub-divided into sub-geographical areas known as networks. Each network contains a base station that communicates with all the mobile stations (e.g., cellular phones) within the network via the air. Systems are labeled with an identification called the system identification (SID), and networks within a system are given a network identification (NID). Therefore, as shown in FIG. 1, every network and associated base station can be identified by an SID/NID pair.
A mobile station stores a list of one or more home SID/NID pairs. The mobile station is categorized as roaming if the mobile station's home SID/NID pair does not match the SID/NID pair of the mobile station's current location. The mobile station is categorized as a foreign NID roamer if one of the stored SIDs matches with the current network's SID. The mobile station is categorized as a foreign SID roamer if none of the stored SIDs match the current network's SID. The mobile station, based on this information along with some parameters stored in its permanent memory, makes a determination about the type of service available when roaming.
Upon power-up, the mobile station attempts to register with its home base station in its home SID/NID pair. Traditionally, the mobile station attempts to access the home mobile station by sequentially scanning a list of frequency channels associated with the home SID/NID pair. However, if the mobile station is roaming, the mobile station will not be able to register with its home base station. If this is the case, the mobile station sequentially scans lists of frequency channels associated with foreign networks. The scanning is traditionally carried out in a sequential manner starting with the foreign networks located within the home system and proceeding through a list of foreign SID/NID pairs. As can be appreciated, the sequential scanning of multiple frequency channels within multiple SID/NID pairs can take a considerable amount of time.
One available time reduction method is disclosed in U.S. Pat. No. 5,517,677 to Moon. The Moon patent is directed to a method of weighting a list of networks within a given system. Using the Moon method, a mobile station maintains an adaptive queue of previously accessed networks. By maintaining the adaptive queue the mobile station can scan frequently visited networks in a weighted fashion since the most frequently visited network should have the greatest number of entries in the adaptive queue. In operation, the first network in the adaptive queue is scanned if the mobile station fails to access a home network. If the mobile station is unable to access the first network in the adaptive queue, the mobile station scans a network that is adjacent to the first network. If the mobile station fails to access the adjacent network, the mobile station scans a first network selected from a universal table of networks. If the mobile station fails to access the network from the universal list of networks, the mobile station repeats its scan of the home network. If the repeated scan fails, the mobile station scans the second network in the adaptive queue of networks. If the scan fails, the mobile station scans a network adjacent to the second network. If the scan fails, the mobile station attempts to access the second network on the universal table. This cyclic method is repeated until a network is accessed or until all the networks on the universal table are scanned.
Although the Moon method is an improvement over the conventional sequential scanning of networks, the Moon method has a number of drawbacks. One drawback is that the scanning sequence in the Moon method only attempts to access a single frequency channel for each scanned network. Another drawback is that the Moon method does not record the network movement of the mobile station. Due to these drawbacks, the scanning sequence in the Moon method can take a considerable amount of time.
In view of the foregoing, it can be seen that there is a need for a method and apparatus that reduces the amount of time that is needed for a mobile station to access a network upon power-up. The present invention provides a method and apparatus to meet this need.